1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an inverted mesa-shaped ridge on its upper surface and to a method of manufacturing such a semiconductor laser.
2. Description of the Related Art
There has been employed a semiconductor light emission device as a light source in information/image processing, communication, measurement, medicine and the like and there have been made various attempts to obtain a high power light beam of a single wavelength.
In a semiconductor laser having an oscillation wavelength of 0.6 to 1.1 .mu.m, improving its beam quality is useful in increasing function of information/image processing. As a method of improving beam quality of a semiconductor laser, it has been proposed to control the laser to oscillate in a single transverse mode. Conventionally, in order to realize oscillation in a single transverse mode, a ridge is formed by etching a multi-layered semiconductor structure including an active layer (light emission layer) short of the active layer by use of an insulating film or the like as a mask and then a current stopping layer is formed by regrowth so that current is poured only into a limited area.
The shape of the ridge is an important parameter which governs properties and production of devices. For example, when the ridge 8 is of a regular mesa shape (a trapezoid tapered outward) as shown in FIG. 8, the region of the semiconductor layer through which an electric current flows is narrower than the light emission region, which increases resistance of the device.
In order to avoid this problem, it is proposed to make the ridge 8 inverted mesa shaped as shown in FIG. 4, thereby reducing resistance of the device.
Though it is easy to form a ridge of an inverted mesa, a (111) crystal face is exposed on one side of the inverted mesa-shaped ridge when the ridge is formed by normal liquid etching. The (111) crystal face exposed on one side of the inverted mesa-shaped ridge makes it difficult to embed a semiconductor layer by regrowth. That is, it has been known that growth of crystal on a (111) face is very difficult and crystal hardly grows on the side of the ridge by a normal method of crystal growth, which makes it difficult to produce devices at high duplicability and high yield.